Repeated exposure or contact with a toxic substance over a long period. The U.S. Environmental Protection Agency defines this period as a significant fraction of the animal's or human's lifetime. The Agency for Toxic Substances and Disease Registry defines chronic exposure as greater than 364 days.

Repeated exposure by the oral, dermal, or inhalation route for more than approximately 10% of the life span in humans (more than approximately 90 days to 2 years in typically used laboratory animal species).